Pattern-transfer process for forming micro-electro-mechanical structures

ABSTRACT

Described is a photolithography “pattern transfer” process for forming Micro-Electro-Mechanical Systems (MEMS) structures. A first material layer is patterned so that raised portions of the layer define features of a MEMS structure to be formed. The resulting pattern is then “transferred” to the surface of a second material layer by etching the top surface of the first material layer, including the raised portions and the valleys defined between the raised portions, until the second layer is exposed between the raised portions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of and claims priority under 35 U.S.C.§119(e) from U.S. patent application Ser. No. 10/032,198 entitled“Multi-Axis Micro-Electro-Mechanical Actuator,” by Vlad J. Novotny andYee-Chung Fu, filed on Dec. 20, 2001, which is a continuation-in-part ofU.S. patent application Ser. No. 09/865,981 entitled “Optical CrossConnect Switching Array System With Optical Feedback,” by Vlad J.Novotny, filed on May 24, 2001, now U.S. Pat. No. 6,483,962, whichclaims benefit of U.S. patent application Ser. No. 60/206,744, entitled“Optical Cross Connect Switching Array Systems With Optical FeedbackControl,” by Vlad J. Novotny, filed May 24, 2000, and Ser. No.60/241,269, filed Oct. 17, 2000. This application additionally relatesto U.S. patent application Ser. No. 10/027,882 entitled “Deep-WellLithography Process For Forming Micro-Electro-Mechanical Structures,” byVlad J. Novotny, Dec. 21, 2001. Each of the above-identified documentsis incorporated herein by reference.

BACKGROUND

As the result of continuous advances in technology, particularly in thearea of networking, such as the Internet, there is an increasing demandfor communications bandwidth. For example, the transmission of images orvideo over the Internet, the transfer of large amounts of data intransaction processing, or videoconferencing implemented over a publictelephone network typically require the high speed transmission of largeamounts of data. As applications such as these become more prevalent,the demand for communications bandwidth will only increase.

Optical fiber is a transmission medium that is well suited to meet thisincreasing demand. Optical fiber has an inherent bandwidth much greaterthan metal-based conductors, such as twisted-pair or coaxial cable; andprotocols such as Synchronous Optical Networking (SONET) have beendeveloped for the transmission of data over optical fibers.

Optical fiber is used to form optical networks that carry data, voice,and video using multiple wavelengths of light in parallel. Light isrouted through the network from its originating location to its finaldestination. Since optical networks do not generally have a singlecontinuous optical fiber path from every source to every destination,the light is switched as it travels through the optical network.Previously, this switching was accomplished usingoptical-electrical-optical (“OEO”) systems, where a light signal wasconverted to an electrical signal, switched electrically, and thenoutput optically. Because in OEO systems the signal must be convertedfrom optical to electrical, switched, then converted back to optical,OEO systems are relatively large, complex, and expensive. Moreseriously, the OEO systems are slower than purely optical systems, andconsequently introduce undesirable bottlenecks.

Much effort is being expended on the development of all-opticalcross-connect switching systems, some of which employ arrays ofelectrostatically, electromagnetically, piezoelectrically, or thermallyactuated mirrors. Digitally controlled mirrors with on and off statescan be used to switch between small numbers of ports while analogcontrolled mirrors can be implemented with a small or a large number ofports. Analog controlled mirrors require bi-axial actuation;unfortunately, most electrostatic actuators used to position thesemirrors suffer from relatively low torque, and consequentially requirerelatively high supply voltages to produce sufficient motion. The lackof torque also renders electrostatic actuators very sensitive tovibrations. There is therefore a need for a bi-axial actuator thatoperates at lower voltages and is relatively insensitive to vibration.

SUMMARY

The invention is directed to Micro-Electro-Mechanical Systems (MEMS)actuators that employ electrostatic comb electrodes to position mirrorsalong multiple axes. In one embodiment, an actuator assembly includes anactuator support, typically a silicon wafer, supporting one or morefixed comb-shaped electrodes, each with a plurality of teeth. A frameflexibly connected to the actuator support includes complementary setsof movable comb electrodes, the teeth of which are arrangedinterdigitally with the teeth of the fixed combs. The frame can betilted with respect to the actuator support along a first fulcrum axisby applying a potential difference between the fixed and movable combs.

Each actuator assembly also includes an actuated member flexiblyconnected to the frame. In the depicted embodiment, the actuated memberis a mirror mount. In other embodiments, the actuated member may supporte.g. a filter, a lens, a grating, or a prism.

The actuated member and the frame include electrically isolated,interdigitated, comb electrodes. The actuated member can be movedrelative to the frame along a second fulcrum axis by applying apotential between the comb on the frame and the comb on the actuatedmember. The actuated member can also be moved translationally byapplying a potential between interdigitated combs.

In one embodiment, the hinges are made using the same conductive layersas the combs. The process used to form the hinges may differ from theprocess used to form the combs. Such processes allow the stiffness ofthe hinges to be adjusted independently. For example, the hinges may bemade thinner to reduce the amount of torque required to move theactuated member. In another embodiment, serpentine hinges are employedto provide still greater flexibility.

A number of novel process sequences can be employed to manufacture MEMSactuators in accordance with the invention. In one such process,referred to herein as a “wafer bonding” process, one device layer on aSilicon-On-Insulator (SOI) or Spin-On-Glass (SOG) wafer is patterned toinclude the combs, hinges, etc., of the MEMS actuator(s) being formed.This patterned layer is then oxide- or glass-bonded to an intrinsicanchor wafer. A via etching is then performed on the other side of theintrinsic anchor wafer to electrically connect the devices to thedriving circuitry. The other side of the original SOI or SOG wafer isthen ground, polished, patterned, and etched as another device layer. Upto four different thicknesses are defined in these lithographicprocesses.

In another process, referred to herein as a “pattern transfer” process,one device layer is patterned to include features similar to the combs,hinges, etc., of the MEMS actuators being formed. The resulting patternis then “transferred” to the surface of a second material layer byetching the top surface of the first material layer—including the raisedportions and the valleys defined between the raised portions—until thesecond layer is exposed between the raised portions.

A third process that can be used to form MEMS actuators in accordancewith the invention, referred to herein as “deep-well lithography,”differs from conventional lithography in that the surface beingpatterned is not the uppermost surface. The focal plane of thephotolithography equipment is offset from the uppermost surface asappropriate to account for the depth of the well in which the pattern isto be formed.

Both the pattern-transfer process and deep-well lithographyadvantageously reduce the number of process steps required to produceMEMS actuators in accordance with the invention, and can additionally beused to form structures other than MEMS actuators.

This summary does not limit the invention, which is instead defined bythe appended claims.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A and 1B, respectively, are plan views of the upper and lowerportions of a two-axis, Micro-Electro-Mechanical System (MEMS) actuatorin accordance with one embodiment of the present invention.

FIGS. 2 through 32 depict a wafer-bonding process sequence in accordancewith an embodiment of the invention.

FIGS. 33A and 33B are plan views depicting the top and bottom halves3300 and 3310 of an actuator in accordance with another embodiment ofthe invention.

FIGS. 34-49 depict an alternate fabrication process, referred to here asthe “pattern transfer” process, that can be used to fabricate MEMSactuators in accordance with the invention.

FIGS. 50-65 depict an alternate fabrication process, referred to here as“deep-well lithography,” that can be used to fabricate MEMS actuators inaccordance with the invention.

FIGS. 66A and 66B depict an optical switch 6600 in accordance with oneembodiment of the invention.

FIGS. 67A and 67B depict a packaging concept for MEMS actuators inaccordance with one embodiment of the invention.

FIGS. 68A and 68B, respectively, are plan views of a top half 6800 and abottom half 6805 of a multi-axis MEMS actuator in accordance withanother embodiment of the invention.

FIGS. 69A and 69B, respectively, are plan views of a top half 6900 and abottom half 6905 of a multi-axis MEMS actuator in accordance withanother embodiment of the invention.

FIGS. 70A and 70B, respectively, are plan views of a top half 7000 and abottom half 7005 of a multi-axis MEMS actuator in accordance withanother embodiment of the invention.

FIGS. 71A and 71B, respectively, are plan views of a top half 7100 and abottom half 7105 of a multi-axis MEMS actuator in accordance withanother embodiment of the invention.

FIGS. 72A and 72B, respectively, are plan views of a top half 7200 and abottom half 7205 of a multi-axis MEMS actuator in accordance withanother embodiment of the invention.

FIGS. 73A and 73B, respectively, are plan views of a top half 7300 and abottom half 7305 of a multi-axis MEMS actuator in accordance withanother embodiment of the invention.

DETAILED DESCRIPTION

FIGS. 1A and 1B, respectively, are plan views of the top (T) and bottom(B) portions of a multi-axis, Micro-ElectroMechanical Systems (MEMS)actuator in accordance with one embodiment of the present invention.Bottom half 100 includes a pair of fixed combs 107 and 109 and a pair ofelectrodes 117 ard 118 attached firmly to the underlying substrate (notshown). Each of fixed combs 107 and 109 includes a respective pluralityof teeth 106B and 108B that extend in the direction depicted ashorizontal in FIGS. 1A and 1B. Fixed combs 107 and 109 are electricallyisolated from one another so that disparate voltages can be appliedthereto. The “B” in numerical designations 106B and 108B indicate thatteeth 106B and 108B are associated with bottom half 100. The remainderof this application follows this convention.

Bottom half 100 includes frame portions 111B and 112B, which alsofunction as frame combs. Frame portions 111B and 112B each support aplurality of frame teeth 113B and 115B, respectively, which extend in adirection perpendicular to the fixed teeth of combs 107 and 109. Frameportions 111B and 112B connect to respective electrodes 117 and 118 viaa pair of hinge portions 119B. Frame portions 111B and 112B, includingteeth 113B and 115B, are disposed above the underlying substrate soframe portions 111B and 112B can pivot along a fulcrum axis FA1 definedalong hinges 119B.

Turning to FIG. 1A, top half 105 is bonded over bottom half 100 with anelectrically insulating layer (detailed below) sandwiched in between.Top half 105 includes a frame 111T (“T” is for “top”) bonded to frameportions 111B and 112B of FIG. 1B. Frame portion 111T includes aplurality of movable combs 106T and 108T, each including a plurality ofcomb teeth extending in the horizontal direction of FIGS. 1A and 1B.Movable comb teeth 106T and 108T are rigidly connected to frame portion111T, and are arranged above fixed combs 106B and 108B such that thefixed and movable teeth are interdigitated from a perspectiveperpendicular to a plane defined by the horizontal and vertical axesdepicted in FIGS. 1A and 1B (i.e., normal to the page).

Top half 105 includes an actuated member 123, in this case a mirrorsurface, connected to frame portion 111T via a pair of hinges 125.Hinges 125 allow member 123 to pivot along a second fulcrum access FA2perpendicular to the fulcrum access FA1 defined by hinges 119B. Member123 additionally includes a collection of combs 113T and 115T, each ofwhich includes a plurality of teeth extending over and in parallel withrespective teeth 113B and 115B of bottom half 100. The teeth in combs113T (and 115T) and teeth 113B (and teeth 115B) are arrangedinterdigitally from a perspective perpendicular to a plane defined bythe vertical and horizontal axes of FIGS. 1A and 1B.

The lower counterparts to hinge portions 119T, depicted in FIG. 1B ashinge portions 119B, electrically connect electrodes 117 and 118 torespective frame-portions 111B (and teeth 113B) and 112B (and teeth115B) so that voltage may be applied to combs 113B and 111B viaelectrodes 117 and 118, respectively. Returning to FIG. 1A, hingeportions 119T and hinges 125 electrically connect combs 113T, 115T,frame portion 111T, and combs 106T and 108T to the surrounding silicon130.

In one embodiment, member 123 is actuated in one direction along FA2axis (say positive direction) by holding silicon 130 (i.e. teeth 106T,108T, 113T and 115T) at ground potential and also teeth 115B, 106B and108B at ground potential while adjusting the voltage levels applied toteeth 113B. Electrical leads that run along hinge portions 119B connectteeth 113B and 115B to the respective electrodes 117 and 118. To movemember 123 in the negative direction, ground potential is kept again atall top teeth, i.e. 106T, 108T, 113T and 115T, and at 108B, 108B and113B, while a desired voltage is applied to teeth 115B. To rotate frame111T along FA1 axis in one direction, all top teeth and bottom teeth113B, 115B, and 108B are at ground potential and teeth 106B have voltageapplied to them; to rotate frame 111T along FA1 in the other direction,all top teeth and bottom teeth 113B, 115B, and 106B are at groundpotential and teeth 108B have voltage applied to them. To rotate member123 along both FA1 and FA2 axes, different voltages are applied to 106Band 113B (or 115B) or to 108B and 113B (or 115B) at the same time.Member 123 may also be moved in a direction normal to the fulcrum axesby applying a potential difference between the combs of top half 105 andbottom half 100. Member 123 may therefore be positioned in threedimensions.

Bottom frame portions 111B and 112B are bonded to top frame portion 111Tduring the process sequence described below. The resulting frame can berotated along the axis FA1 defined by hinges 119B and 119T by applying avoltage difference between teeth 106B and ground or between teeth 108Band ground. Combs 106T and 108T are termed “movable” because they moverelative to stationary combs 106B and 108B. Similarly, actuated member123 can be rotated along axis FA2 by applying a voltage differencebetween the silicon 130 and either electrode 117 or electrode 118.

FIGS. 2 through 32 depict a process sequence in accordance with anembodiment of the invention. The process sequence can be employed tofabricate an actuator of the type depicted in FIGS. 1A and 1B. FIGS. 2through 32 depict the device in cross section, with the resultingstructure appearing similar to the device of FIGS. 1A and 1B cut alongline A-A′.

FIG. 2 depicts an SOI or SOG wafer 200 that includes a layer of handlesilicon 205 connected to a 20-100 micron thick device silicon layer 210via a 1-2 micron thick silicon dioxide or glass layer 215. As depictedin FIG. 3, the exposed surfaces of silicon layers 205 and 210 are coatedwith silicon dioxide mask layers 300 and 305. The resulting structure isthen masked using a photoresist layer 400 (FIG. 4) to define a set ofalignment marks 405. Alignment marks 500 are then etched in oxide layer300 and the photoresist layer 400 is removed to produce the structure ofFIG. 5. Silicon layers 205 and 210 are both doped, either n-type orp-type, and have a resistivity of about 5 to 100 ohms-cm in oneembodiment.

Next, a layer of photoresist is patterned over oxide layer 305 to createa mask 600 used to define each element of bottom half 100 (FIG. 1B)except for hinge portions 119B. The exposed portions of oxide layer 305are then subjected to a dry silicon-dioxide etch, leaving an oxide mask700 of the pattern defined by mask 600. Mask 600 is then removed,leaving the structure of FIG. 7.

A photoresist layer 800 is patterned over oxide mask 700 and over thoseportions of device silicon 210 that are to become hinge portions 119B(FIG. 8). The resulting structure is then subjected to a siliconreactive-ion etch (RIE) to remove a desired depth of device layer 210 inthe exposed regions (FIG. 9). The mask used in this etch step includestwo sub-masks: oxide mask 700 and the pattern photoresist layer 800. Theetch depth is related to the final thickness of hinge portions 119B. Thephotoresist mask 800 is then removed, exposing oxide mask 700 and theportions of device layer 210 that will become hinge portions 119B (FIG.10).

A second silicon RIE removes the remaining unmasked silicon of layer 210down to oxide or glass layer 215, which acts as an etch-stop layer (FIG.11). Portions 1100 of silicon layer 210 that will later become hingeportions 119B are left adhered to oxide layer 215 because, as shown inFIG. 10, portions 1100 entered the etch step thicker than thesurrounding exposed portions of silicon layer 210. The hinges undergothis fabrication sequence to make them thinner, and consequently moreflexible, than the surrounding device features. When it is desired tokeep hinges of the same thickness as the teeth, the steps of FIGS. 8-10are skipped.

In an optional step, a refractory coating 1200 is applied through ashadow mask to an exposed portion of oxide layer 215 to balance thestress imposed by a reflective layer applied opposite coating 1200 onlayer 205 in a later step. The resulting structure is depicted in FIG.12.

The process of fabricating the actuator support begins, as shown in FIG.13, with an intrinsic silicon wafer 1300 coated with a layer of silicondioxide (or glass) 1302. Layer 1302 is conventionally masked using alayer of photoresist 1400 to define electrical contacts to silicon layer1300 and a plurality of alignment marks (FIG. 14). Layer 1302 is shownin FIG. 15 to include an area 1500 in which will be formed a via and anumber of openings 1502.

In FIGS. 16 and 17, a photoresist layer 1600 is patterned over silicon1300 to define an area 1700 in which approximately 100 microns ofsilicon is etched away from silicon layer 1300 using RIE. The resultingstructure, including area 1700 and alignment marks 1502, is depicted inFIG. 17.

In the next step, the structure of FIG. 17 is brought into contact withthe structure of FIG. 12, out of which will be formed bottom and tophalves 100 and 105 (FIG. 18). The two portions are aligned using therespective alignment marks 400 and 1502 and then fused together using aheat treatment. In an embodiment in which mask 700 is silicon dioxide,the structure is heated to approximately 1,000 to 1,100 degrees Celsius.In an embodiment in which mask 700 is sol gel glass, the structure isheated to between 200 and 400 degrees Celsius. The lower processtemperatures employed when glass is used for layers 215 and mask 700minimize stresses associated with thermal expansion in the multi-layerstructures.

Referring now to FIG. 19, the top surface of silicon layer 205 isground, lapped, and polished to a mirror finish. The resulting thinnedsilicon layer 205 is approximately 20 to 100 microns thick. Next (FIG.20), a photoresist mask 2000 is applied to oxide layer 1302 using thesame mask used to define the pattern of oxide layer 1302 in FIG. 14. Acombined mask of photoresist (2000) and oxide (1302) is used for verydeep silicon etching. A subsequent silicon RIE step removes some oflayer 1300 in the vicinity 1500 to expose a portion of oxide mask 700,producing the structure of FIG. 21. The RIE used to form the structureof FIG. 21 is adjusted so that the sidewalls of opening 2100 are notnormal to the surface. Alternatively, wet etching of silicon can be usedto produce sloping wall vias. In this case, silicon nitride mask ispreferable to silicon dioxide mask. This leaves alignment marks 2102 inlayer 1300 but prevents those marks from extending far into layer 1300.

In the next step, oxide layer 1302 and the portion of oxide layer 700exposed during the previous step are removed using an oxide dry-etchprocess. In the resulting structure, illustrated in FIG. 22, theunderside of layer 210 is exposed to allow a subsequently formed via tomake electrical contact to the portion of layer 210 that will become thebody of comb 109 of FIG. 1A. Similar vias make contact to electrode 117,electrode 118, and comb 107, though these are not shown in this crosssection.

Continuing to FIG. 23, another oxide layer 2300 is formed on the topsurface of silicon layer 205 using either chemical vapor deposition orsputter deposition. The pads (not shown) and vias, one of which isdepicted in FIG. 24, are then metalized using a conventionalmetalization process that employs a shadow mask. Via 2400 contacts theunderside of silicon layer 210 at a portion that will become the body ofcomb 109 (FIG. 1B) of the bottom half of the actuator under fabrication.

Most of the features of bottom layer 100 of FIG. 1B have been defined atthis stage in the process sequence. The process of patterning thestructures required to form top half 105 begins with a photoresist mask2500 depicted in FIG. 25. The upper surface of oxide layer 2300 is dryetched through mask 2500 to expose the underlying silicon layer 205. Theresulting structure is depicted in FIG. 26.

Turning to FIG. 27, a photoresist layer 2700 is applied over eachfeature of the oxide mask patterned in layer 2300, and additionally overthose portions of silicon layer 205 that will form hinges 125 and hingeportions 119T. Those portions can be identified in FIG. 27 as theportions of photoresist layer 2700 deposited directly on the surface ofsilicon layer 205. The top surface of the resulting structure is thensubjected to a silicon RIE process that removes a desired thickness ofthe exposed portions of silicon layer 205. This etch step defines thethickness of the upper half of hinges 125 and hinge portions 119T, theportions depicted in upper half 105 of FIG. 1A. The resulting structureis depicted in FIG. 28.

Patterned mask layer 2700 is then removed (FIG. 29). Another RIE thenremoves the remaining silicon in the thinned portions of silicon layer205. As shown in FIG. 30, those portions of silicon layer 205 protectedfrom the first RIE step of FIG. 28, being thicker than the other etchedportions of layer 205, leave features 3000 to form the upper portion ofhinges 125 and hinge portions 119T. Like structures 1100, which form thebottom half of the hinges, structures 3000 are formed thinner thanadjacent elements to adjust hinge flexibility. The resulting structureis subjected to a silicon-dioxide etch to remove oxide layer 2300 andthose portions of oxide layer 215 that connect adjacent elementsdepicted in the cross section of FIG. 30, thereby producing thestructure of FIG. 31.

Alternatively, oxide layer 2300 is removed through a shadow mask thatallows oxide etching over the whole surface except the portion that willbecome actuated member 123. In the resulting embodiment, the actuatedmember is coated with an oxide layer on both principal surfaces tominimize mirror distortion. Finally, a reflective surface (a mirror)3200 is added to silicon layer 205. In this case, mirror 3200 is formedby depositing first chromium and then gold onto layer 205 through ashadow mask. The completed structure, illustrated in FIG. 32, isannotated using the numbers introduced in FIGS. 1A and 1B to identifythe actuator structures shown in the cross section in FIG. 32. Onefeature not shown in FIGS. 1A and 1B is the actuator support formed fromsilicon layer 1300.

As is apparent from FIG. 32, teeth 106T (115T) and the underlying teeth106B (115B) appear interdigitated from a perspective normal to mirror3200, but not from a perspective normal to the cross section of FIG. 32.However, the teeth can be drawn toward one another, and thereforeactually interdigitated, by applying a sufficient voltage between theupper and lower teeth. The ability to interdigitate the opposing teethminimizes the clearance, increases the efficiency, and reduces thevoltage required to produce a desired deflection angle.

The cross section of FIG. 32 differs slightly from what would beobtained along line A-A′ of FIGS. 1A and 1B. For example, the number ofcomb teeth differs, and the layers and patterns are not to scale. Suchvariations are commonly used to simplify the description of the process,as is well understood by those of skill in the art. In an actualembodiment, combs 113, 115, 106, and 108 might have 10-100 teeth, forexample, and the teeth might be 5-20 microns wide and 200-500 micronslong.

FIGS. 33A and 33B are plan views depicting the respective top and bottomhalves 3300 and 3305 of an actuator in accordance with anotherembodiment of the invention. The actuator depicted in FIGS. 33A and 33Bis functionally similar to the one depicted in FIGS. 1A and 1B. However,the structure of FIGS. 33A and 33B employs a different combconfiguration, as is obvious from the plan views, and also includes moreflexible serpentine hinges. The serpentine hinges can be made the samethickness as other elements (e.g., the comb teeth), or can be madethinner using the process shown in connection with FIGS. 2 through 32.

The distance from the tip of teeth 3306, 3308, 3313 and 3315 to theirrotational axes are longer than in the embodiment of FIGS. 1A and 1B.Therefore, the torque generated by the same voltage difference isincreased. Mirror teeth 3313T and 3315T with variant teeth length areattached to the mirror directly and to surrounding silicon 3330 viahinges 3325, frame 3311T, and hinge portions 3319T. Variable teethlength is important for linearization of voltage response and damping ofresonances. Frame teeth 3313B and 3315B are arranged interdigitally withmirror teeth 3313T and 3315T and connected to electrodes 3317 and 3318independently through hinge portions 3319B. A voltage difference can beapplied between 3313T and 3313B or between 3315T and 3315B to rotate themirror with respect to the frame in the axis defined by hinges 3325. Theframe teeth 3306T and 3308T are also arranged interdigitally with thestationary comb teeth 3306B and 3308B to rotate the mirror/frame withrespect to the axis defined by the hinges formed of top and bottom hingeportions 3319T/B. Two separated frame portions 3316 are designed toincrease the frame rigidity without increasing the electrostaticcoupling between different sets of teeth, 3306 and 3313. Also important,the actuator is designed so that the reflective surface 3323 is as greata percentage of the total actuator area (including the exposed portionsof the actuator support) as practical, which is over 25% in the depictedembodiment.

FIGS. 34-49 depict an alternate fabrication process, referred to here asthe “pattern transfer” process, that can be used to fabricate MEMSactuators in accordance with the invention. FIGS. 34-49 depict thedevice in cross section, with the resulting structure appearing similarto the device of FIGS. 1A and 1B cut along line A-A′.

FIG. 34 depicts a wafer 3400 that includes a layer of handle silicon3405 connected to a 20-100 micron thick device silicon layer 3410 via a1-2 micron thick layer 3415 of silicon dioxide or spin-on glass. Asdepicted in FIG. 35, the exposed surfaces of silicon layers 3405 and3410 are coated with silicon dioxide mask layers 3500 and 3505. Siliconlayers 3405 and 3410 are both doped, either n-type or p-type, and have aresistivity of about 5 to 100 ohms-cm in one embodiment.

A layer of photoresist is patterned over each of respective oxide layers3500 and 3505 to create a pair of masks 3600 and 3610 (FIG. 36). Theexposed portions of oxide layers 3500 and 3505 are then subjected to adry silicon-dioxide each, leaving oxide masks 3700 and 3710. Masks 3600and 3610 are then removed, and the upper surface of the resultingstructure is subjected to another photolithographic patterning step thatforms the hinge patterns 3805, leaving the structure of FIG. 38.Optionally, layer 3505 can be a metal film, such as aluminum, chromium,or titanium, and mask 3610 can be formed of oxide. In such embodiments,the metal layer is etched using metal etches rather than oxide etches.

Next, a silicon RIE removes a desired depth of device layer 3410,leaving the structure of FIG. 39. Mask 3800 is then removed, leaving thestructure of FIG. 40. A second silicon RIE then removes the remainingunmasked silicon of layer 3410 down to oxide layer 3415, which acts asan etch-stop layer (FIG. 41). Portions 4100 of silicon layer 3410 thatwill later become hinges 125 and hinge portions 119T are left adhered tooxide layer 3415. The hinges undergo this fabrication sequence to makethem thinner, and consequently more flexible, than surrounding devicefeatures. When it is desired to keep the hinges of the same thickness asthe teeth, the steps of FIGS. 38-42 are skipped.

In the next step, a photoresist layer 4200 is patterned over the oxidemask 3710, with the addition of the portion 4205 that masks what willbecome hinge portion 119B (FIG. 42). Next, the lower surface of siliconlayer 3405 is subjected to a silicon RIE that removes a desiredthickness of the exposed portions of silicon layer 3405. This etch stepdefines the thickness of hinge portions 119B of FIG. 1B. The resultingstructure is depicted in FIG. 43. The photomask is then removed, leavingthe structure of FIG. 44, which includes a raised element 4400.

Another silicon RIE then removes a second desired thickness of theexposed portions of silicon layer 3405. This step defines the thicknessof what will become the comb teeth of bottom portion 100 of the actuatorof FIGS. 1A and 1B. FIG. 45 depicts the resulting structure.

Another photoresist mask 4600 is added by spray coating to protectportions of oxide mask 3710 (FIG. 46); the exposed portions of oxidemask 3710 are then removed using a dry silicon-dioxide etch step. FIG.47 depicts the resulting structure. The remaining silicon in the thinnedportions of silicon layer 3405 is then removed using another RIE, withoxide layer 3415 acting as an etch-stop layer. As shown in FIG. 48,those portions of silicon layer 3405 protected from previous RIE steps,being thicker than the other etched portions of layer 3405, leavefeatures 4800 and 4805. Features 4800 and 4805 will form the bottomcombs (115B and 106B) and hinge portion 119B, respectively, of FIG. 1B.

Finally, the structure of FIG. 48 is subjected to a silicon-dioxide etchto remove oxide layers 3700 and 3710, and to remove those portions ofoxide layer 3415 that connect adjacent elements depicted in the crosssection of FIG. 48. Though not shown, a reflective surface issubsequently added to silicon layer 3410. The completed actuator,illustrated in FIG. 49, is annotated using some of the numbersintroduced in FIGS. 1A and 1B to identify the actuator structures shownin the cross section. As with the previous example of FIG. 32, the crosssection of FIG. 32 differs slightly from what would be obtained alongline A-A′ of FIGS. 1A and 1B. What remains of silicon layer 3405 formsthe actuator support.

As noted above, the process of FIGS. 34-49 is referred to as a “patterntransfer” process. The name “pattern transfer” refers to the steps bywhich a pattern is formed on one surface and transferred to another.Such a pattern transfer is shown, for example, in FIGS. 41-48. In FIGS.41-47, the bottom surface of silicon layer 3405 (a first material layer)is patterned to include features similar to the combs, hinges, etc., ofbottom half 100 of the MEMS actuator of FIGS. 1A and 1B. This pattern isthen “transferred” to the bottom surface of a second material layer,oxide layer 3415 (FIG. 48), by etching silicon layer 3405 until oxidelayer 3415 is exposed between elements of the pattern. The originalelements of the pattern, shown in e.g. FIG. 47, are wholly or partiallyconsumed in the etch process that culminates in the structure of FIG.49.

FIGS. 50-65 depict an alternate fabrication process, referred to hereinas “deep-well lithography,” that can be used to fabricate MEMS actuatorsin accordance with the invention. FIGS. 50-65 depict the device in crosssection, with the resulting structure appearing similar to the device ofFIGS. 1A and 1B cut along line A-A′.

FIG. 50 depicts a wafer 5000 that includes a layer of handle silicon5005 covered with a silicon-dioxide layer 5010, a device silicon layer5015, a second silicon-dioxide layer 5020, and a second device siliconlayer 5025. Device silicon layers 5015 and 5025 are each about 20-100microns thick; oxide layers 5010 and 5020 are each between one and twomicrons thick. Silicon layers 5005, 5015, and 5025 are doped, eithern-type or p-type, and have a resistivity of about 5 to 100 ohms-cm inone embodiment.

As depicted in FIG. 51, the exposed surfaces of silicon layers 5005 and5025 are coated with respective silicon dioxide mask layers 5105 and5100. Next, a layer of photoresist is patterned over each of respectiveoxide layers 5100 and 5105 to create a pair of masks 5200 and 5210 (FIG.52). The exposed portions of oxide layers 5100 and 5105 are thensubjected to a dry silicon-dioxide etch, leaving oxide masks 5300 and5310. Masks 5200 and 5210 are then removed, and another photoresistlayer 5400 is patterned over the oxide mask 5300 with the additionalpatterns 5405 that are to become hinges 125 and hinge portions 119T(FIG. 54).

Next, the upper surface of the resulting structure is subjected to asilicon RIE to remove a desired thickness of device layer 5025, leavingthe structure of FIG. 55. The photoresist layer 5400 is then removed,leaving the structure of FIG. 56.

A second silicon RIE removes the remaining unmasked silicon of layer5025 down to oxide layer 5020, which acts as an etch-stop layer (FIG.57). Portions 5700 of silicon layer 5025 that will later become hinges125 and hinge portions 119T are left adhered to oxide layer 5020. Thehinges undergo this fabrication sequence to make them thinner, andconsequently more flexible, than the surrounding device features. As inthe previous examples, several of the foregoing steps can be eliminatedif the hinges need not be thinner than surrounding device features.

In the next step, another RIE removes the unmasked portion of siliconlayer 5005 down to oxide layer 5010, which acts as an etch-stop layer(FIG. 58). Turning to FIG. 59, a photoresist mask 5900 is then appliedby spray coating to oxide layer 5010 before the lower surface of siliconlayer 5010 is subjected to a dry silicon-dioxide etch process thatremoves exposed portions of oxide layer 5010 to form a mask 6000 (FIG.60). Photoresist mask 5900 is then removed, and another photoresistlayer 6100 is patterned over the oxide mask 6000, with the addition ofthe portion 6105 that will become hinge portion 119B. The resultingstructure is depicted in FIG. 61.

Next, as shown in FIG. 62, a desired thickness of the exposed portionsof silicon layer 5015 is etched away using an RIE. This etch stepdefines the thickness of hinge portions 119B of FIG. 1B. The resultingstructure, after removing photoresist 6100 (FIG. 63), includes anelement 6300.

Another RIE removes the remaining silicon in the thinned portions ofsilicon layer 5015, with oxide layer 5020 acting as an etch-stop layer.As shown in FIG. 64, the portion of silicon layer 5015 protected fromprevious reactive-ion etching, being thicker than the other etchedportions of layer 5015, leaves feature 6400 that will form hinge portion119B of FIG. 1B. Once again, several of the foregoing steps can beeliminated if the hinges need not be thinner than surrounding devicefeatures.

Finally, the structure of FIG. 64 is subjected to a silicon-dioxide etchto remove oxide layers 5300 and 5310, and to remove those portions ofoxide layer 5020 that connect adjacent elements depicted in the crosssection of FIG. 65. Though not shown, a reflective surface is then addedto silicon layer 3410. The completed actuator is annotated using some ofthe numbers introduced in FIGS. 1A and 1B to identify the actuatorstructures shown in the cross section. As with the previous example ofFIG. 32, the cross section of FIG. 65 differs slightly from what wouldbe obtained along line A-A′ of FIGS. 1A and 1B. What remains of siliconlayer 5005 provides the actuator support.

As noted above, the process of FIGS. 50-65 is referred to as “deep-welllithography.” The name refers to the steps by which a pattern is formedupon a surface that is below the uppermost surface of the structurebeing fabricated (i.e., in a well). Such a process is shown, forexample, in FIGS. 59-64, during which silicon layer 5015 is patterned toform features of bottom half 100 of the MEMS actuator of FIGS. 1A and1B.

Deep-well lithography differs from conventional lithography in that thesurface being patterned is not the uppermost surface. The focal plane ofthe photolithography equipment must therefore be offset as appropriateto account for the depth of the well in which the pattern is to beformed. To form mask 5900 of FIG. 59, for example, the photolithographyequipment is first focused on the top surface of oxide layer 5310 todefine the well within which mask 5900 will be formed. The focal planeof the photolithography equipment is then adjusted to account for thecombined thickness of silicon layer 5005 and oxide layer 5310 so thatthe exposure pattern is focused on the portion of mask layer 5900 incontact with oxide layer 5010. The offset can take into account thethickness of a material layer of uniform composition, or a materiallayer made up of two or more sub-layers (e.g., oxide layer 5310 andsilicon layer 5005).

FIGS. 66A and 66B depict an optical switch 6600 in accordance with oneembodiment of the invention. Switch 6600 includes a nine-by-nine mirrorarray 6605 hermetically sealed within a package 6610. Package 6610protects the very fragile mirror array 6605 from physical and chemicalhazards (e.g., dust and condensation), which can easily damage sensitiveMEMS structures or interfere with device operation. Package 6610 ispreferably assembled in an inert, low humidity environment.

Within package 6610, array 6605 is mounted on an integrated circuit 6615that includes the requisite circuitry for controlling array 6605. Array6615 is, in turn, mounted on a ceramic substrate 6620. Package 6610 issealed using a window 6625, both primary surfaces of which includenon-reflective coatings. A heat sink 6630 affixed to substrate 6620dissipates heat generated by circuit 6615. A collection of feed-throughpins 6635 conveys external signals, including power and ground, tocircuit 6615.

Array 6605 includes 81 mirrors, and each mirror requires a number ofelectrical contacts. Other implementations will have more or fewermirrors, and consequently require more or fewer electrical contacts. Asthe number of contacts increases, wirebond pad pitch limitations make itincreasingly difficult to convey a sufficient number of control signalsbetween circuit 6615 and array 6605. “Flip-chip” technology is used insome embodiments to solve this problem. For more information aboutflip-chip technology, see “Flip Chip Challenges,” by Steve Bezuk,Applied Technology Development and Flip Chip, Kyocera America, Inc.,which was first published in HDI Magazine, February 2000, and isincorporated herein by reference.

FIG. 67A depicts an application-specific integrated circuit (ASIC) 6700that includes a collection of contact bumps 6705. FIG. 67B shows ASIC6700 in cross-section along line A-A′ of FIG. 67A, and additionallyshows a portion of a MEMS actuator 6710 with electrical contacts (vias)6715 positioned over and in contact with bumps 6705. Bumps 6705 can beconductive bonding material, such as solder or conductive epoxy;alternatively, bumps 6705 can be replaced with an anisotropic conductivefilm, provided MEMS actuator 6710 is sufficiently robust to withstandthe compressive force required to make effective electrical contactthrough such material.

FIGS. 68A and 68B, respectively, are plan views of a top half 6800 and abottom half 6805 of a multi-axis MEMS actuator in accordance withanother embodiment of the invention. Top half 6800 includes a frame 6810supporting an actuated member 6815. Frame 6810 includes a number ofcurved, moveable combs 6820 interdigitated with a corresponding numberof fixed combs 6825. The actuator of FIGS. 68A and 6813 is similar tothe actuator in FIGS. 1A and 1B, but additionally affords the ability torotate member 6815 in the X-Y plane (FIG. 68B). The actuator of FIGS.68A and 68B can be fabricated using any of the process sequencesdescribed above.

FIGS. 69A and 69B, respectively, are plan views of a top half 6900 and abottom half 6905 of a multi-axis MEMS actuator in accordance withanother embodiment of the invention. Top half 6900 includes a frame 6910supporting an actuated member 6915. Frame 6910 includes a number ofmoveable combs 6920 interdigitated with a corresponding number of fixedcombs 6925. The actuator of FIGS. 69A and 69B is similar to the actuatorin FIGS. 1A and 1B, but affords the ability to translate member 6915linearly along the X and Z axes (FIG. 69B) and rotationally around the Xand Y axes. The actuator of FIGS. 69A and 69B can be fabricated usingany of the process sequences described above.

FIGS. 70A and 70B, respectively, are plan views of a top half 7000 and abottom half 7005 of a multi-axis MEMS actuator in accordance withanother embodiment of the invention. The actuator of FIGS. 70A and 70Bis similar to the actuator of FIGS. 1A and 1B, but includesnon-perpendicular fulcrum axes FA1 and FA2. The actuator of FIGS. 70Aand 70B can be fabricated using any of the process sequences describedabove.

FIGS. 71A and 71B, respectively, are plan views of a too half 7100 and abottom half 7105 of a multi-axis MEMS actuator in accordance withanother embodiment of the invention. Top half 7100 includes four sets ofcombs 7110 interdigitated with four separate fixed combs 7115 on bottomhalf 7105. An actuated member 7120 suspended by four bending, serpentinehinges 7125 can pivot along either of two fulcrum axes FA1 and FA2, orcan be moved vertically along the Z axis normal to the plane defined bythe two fulcrum axes. Advantageously, the actuators described inconnection with FIGS. 71A/B, can be fabricated using fewer process stepsthan other embodiments described herein. The simplified process sequenceis similar to the process described in connection with FIGS. 2-32, buteliminates the need for the steps described in connection with FIGS.8-10 and 14-17. Also advantageous, this embodiment eliminates the needto align two patterned wafers before bonding; instead, an unpatternedwafer is bonded to a patterned wafer. The structures disclosed below anddescribed in connection with FIGS. 72A/B and 73A/B afford the sameadvantages.

FIGS. 72A and 72B, respectively, are plan views of a top half 7200 and abottom half 7205 of a multi-axis MEMS actuator in accordance withanother embodiment of the invention. Top half 7200 includes three setsof combs 7210 interdigitated with three separate fixed combs 7215 onbottom half 7205. By supplying different voltages on selected ones offixed combs 7215, an actuated member 7220 can be tilted in an X-Y planeand can be moved along a Z axis normal to the X-Y plane.

FIGS. 73A and 73B, respectively, are plan views of a top half 7300 and abottom half 7305 of a multi-axis MEMS actuator in accordance withanother embodiment of the invention. Top half 7300 includes a frame 7310supporting an actuated member 7315. Frame 7310 includes a number offrame teeth 7320 interdigitated with corresponding fixed teeth 7325 onbottom half 7305; likewise, member 7315 includes a number of memberteeth 7330 interdigitated with corresponding fixed teeth of combs 7335on bottom half 7305. The actuator of FIGS. 73A and 73B is similar to theactuator in FIGS. 1A and 1B, and affords the ability to rotate member7315 along a first rotational axis defined by torsional hinges 7340, asecond rotational axis defined by torsional serpentine hinges 7345, andtranslationally along the Z axis normal to the two rotational axes.

The foregoing embodiments include springs that lie in substantially thesame plane as the actuated member. It is also possible to attach anactuated member to a member support using one or more flexible elementsextending from the bottom of the actuated member. In the case of amirror, such a structure might be similar to a table on one or moreflexible legs. The table surface (the mirror) would be movable in atleast two dimensions. Such a structure could be fabricated using e.g.LIGA micromachining technology (“LIGA” is an acronym from German wordsfor lithography, electroplating, and molding).

For additional information relating to MEMS actuators in general, andoptical cross-connect switches in particular, see the following U.S.patent applications, each of which is incorporated by reference;

-   -   1. Ser. No. 09/880,456, entitled, “Optical Cross Connect        Switching Array System With Electrical And Optical Position        Sensitive Detection,” by Vlad Novotny, filed Jun. 12, 2001; and    -   2. Ser. No. 09/981,628, entitled “Micro-Opto-Electro-Mechanical        Switching System,” by Vlad J. Novotny and Parvinder Dhillon,        filed on Oct. 15, 2001.

While the present invention has been described in connection withspecific embodiments, variations of these embodiments will be obvious tothose of ordinary skill in the art. For example, each fulcrum axis maybe provided along an edge of the actuated member and the number of combsmay be different. Therefore, the spirit and scope of the appended claimsshould not be limited to the foregoing description.

1. A photolithographic process sequence for manufacturing MEMSstructures from a first material layer of a first-material-layerthickness disposed over and in contact with a second material layer, thesequence comprising: a. forming a mask over the first material layer,wherein the mask leaves portions of the first material layer exposed; b.etching the first material layer in the exposed portions to a firstdepth less than the first-material-layer thickness, wherein the maskedportions form a raised pattern defined by recessed areas formed in theexposed portions; c. removing at least a portion of the mask, leaving atleast a portion of the raised pattern and the recessed areas exposed;and d. etching the exposed raised pattern and recessed areas of thefirst material layer until the second material layer is exposed in therecessed areas, leaving the pattern affixed to the second materiallayer.
 2. The method of claim 1, wherein the pattern comprises first andsecond portions and wherein forming the mask comprises: a. forming afirst sub-mask defining the first portion of the pattern; and e. forminga second sub-mask over the first sub-mask, the second sub-mask definingthe second portion of the pattern.
 3. The method of claim 2, whereinremoving at least a portion of the mask comprises removing the secondsub-mask.
 4. The method of claim 3, wherein the second sub-maskcomprises photoresist.
 5. The method of claim 1, further adapted tomanufacture a second collection of MEMS structures from a third materiallayer of a third-material-layer thickness, the sequence furthercomprising: a. forming a second mask over the third material layer,wherein the second mask leaves portions of the third material layerexposed; b. etching the third material layer in the exposed portions ofthe third material layer to a second depth less than thethird-material-layer thickness, wherein the masked portions of the thirdmaterial layer form a second raised pattern defined by recessed areasformed in the exposed portions of the third material layer; c. removingat least a portion of the second mask, leaving at least a portion of thesecond raised pattern and the recessed areas in the third material layerexposed; and d. etching the exposed second raised pattern and recessedareas in the third material layer to remove the material in the recessedareas of the third material layer.
 6. The method of claim 5, whereinsubstantially all of the material in the recessed areas of the thirdmaterial layer is removed.
 7. The method of claim 5, wherein the secondmaterial layer is disposed between the first material layer and thethird material layer.
 8. A photolithographic method of patterning afirst material layer over a second material layer, the first materiallayer being of a thickness and having a first surface in contact withthe second material layer and a second surface, the method comprising:a. forming a mask over the second surface of the first material layer,wherein the mask leaves portions of the second surface exposed; b.etching the first material layer in the exposed portions to a firstdepth less than the thickness of the first material layer, wherein themasked portions form a raised pattern defined by recessed areas formedin the exposed portions; c. removing the mask, leaving the raisedpattern and the recessed areas exposed; and d. etching the raisedpattern and recessed areas of the first material layer until the secondmaterial layer is exposed in the recessed areas, leaving the patternaffixed to the second material layer.
 9. The method of claim 8, whereinthe first material layer comprises a semiconductor.
 10. The method ofclaim 9, wherein the second material layer comprises an insulator. 11.The method of claim 8, wherein the first material later comprises asemiconductor, and wherein the second material layer comprises aninsulator.
 12. The method of claim 8, wherein the mask comprises asemiconductor.
 13. The method of claim 8, wherein at least one of theetchings are accomplished using a reactive ion etch process.
 14. Amicro-machining method for patterning a first material layer over asecond material layer, the first material layer being of a thickness andhaving a first surface in contact with the second material layer and asecond surface, the method comprising: a. forming a first mask over thesecond surface of the first material layer, wherein the first maskleaves portions of the second surface exposed; b. etching the firstmaterial layer in the exposed portions to a first depth less than thethickness of the first material layer, wherein the masked portions forma raised pattern defined by recessed areas formed in the exposedportions; c. forming a second mask over a first portion of the raisedpattern, leaving a second portion of the raised pattern and the recessedareas exposed; and d. etching the second portion of the raised patternand recessed areas of the first material layer until the second materiallayer is exposed in the recessed areas, leaving the pattern affixed tothe second material layer, wherein the second mask protects the firstportion of the raised pattern from the etching of (d), leaving thesecond portion of the raised pattern thinner than the first portion ofthe raised pattern.
 15. The method of claim 14, wherein the second maskcomprises at least a portion of the first mask.
 16. The method of claim14, wherein the first mask comprises photoresist.
 17. The method ofclaim 14, wherein the first material layer comprises a semiconductor andthe second material layer comprises an insulator.
 18. The method ofclaim 14, further comprising removing at least a portion of the secondmaterial layer.